The electric drives mounted on the actuators of pipe valves must be provided with limiters of the axial force (torque) transmitted to the actuator. It is generally preferred that this axial force limiter be available both when opening the stop valves and when closing the same. This necessitates the provision of a pair of limiters, resulting in increased overall dimensions of the electric drive.
Known in the art is an electric drive comprising an electric motor, a lead screw coupled to the actuator and connected with the shaft of the electric motor by a helical gearing, an axial force limiter formed by a pair of springs disposed along the electric motor shaft, with the drive gear of said gearing interposed therebetween and capable of being axially displaced. Each of said springs is connected with the corresponding microswitch (DE, C, 155950).
With an excessive axial force acting on the lead screw, the displacement of the screw is slowed down, and the driven gear of the gearing mounted thereon also slows down its rotation, while the drive gear keeps rotating. Due to the force component directed along the axis, the drive gear moves axially to either side, depending on the direction of the lead screw motion, against the pressure of the corresponding spring which is compressed to open the corresponding microswitch, and the electric motor is stopped.
Said electric drive suffers from a disadvantage consisting in its larger size as a result of the two springs provided in the axial force limiter and disposed along the electric motor shaft, for two opposite moving directions of the lead screw.
Known is an electric drive comprising a base, an electric motor including a frame with a stator and a hollow rotor mounted therein, a lead screw located in the hollow rotor, an axial force limiter comprising a washer and a resilient member with one end thereof contacting the washer, an assembly kinematically coupled with the axial force limiter and capable of being axially displaced, the axial force limiter being interposed between the bearing surface of the base and the pressure surface of the assembly kinematically coupled with the axial force limiter (FR, B, 2274859). In this apparatus, the assembly kinematically coupled with the axial force limiter is represented by a sliding nut locked against rotation relative to the rotor by means of a key and capable of being axially displaced, to some extent, with respect to the rotor. The extent of this displacement is limited by a pair of axial force limiters disposed on either side of the sliding nut, in its moving direction. When the axial force transmitted to the actuator is exceeded, the lead screw stops, and the sliding nut working against the force of the resilient member in a particular axial force limiter (depending on the direction of the lead screw motion) starts moving axially along the thread of the lead screw that has stopped, with respect to the rotor, thus limiting the axial force transmitted to the actuator.
One drawback of the apparatus described consists in considerable longitudinal dimensions of the electric drive due to the presence of two axial force limiters disposed on either side of the sliding nut. Besides, the two axial force limiters present in the apparatus result in its complicated design.